Bad sectioning of item/enemy routes #1 and #2 (zero-based numbering). This trivial route split was supposed to end on the first ramp; but instead, both routes persist redundantly for a while, and end up dependently taking different routes on the next important junction where you either turn 90° or go straight. Thus, Bullet Bill may fail there if you use it after entering the "wrong" route (depending on the route you took earlier; reset after a respawn).